1. Field of the Invention
The present invention relates to a carrier for use in two component developers used for electrophotography and electrostatic recording. The present invention also relates to a developer for developing electrostatic latent images, which uses the carrier. Further, the present invention relates to an image forming method, an image forming apparatus, and a process cartridge, which form a visual image using the developer.
2. Discussion of the Background
In dry developing methods used for electrophotography and electrostatic recording to visualize electrostatic latent images, a powder toner, which has been charged by friction with a frictional charge imparting member, is electrostatically adhered to an electrostatic latent image, resulting in formation of a visual image.
Such dry developing methods are broadly classified into one component developing methods using a one component developer including a toner as a main component, and two component developing methods using a two component developer including a mixture of a toner and a carrier such as glass beads, particulate magnetic materials, and coated carriers including core particles whose surface is coated with a resin or the like.
In such two component developers, toner particles are born on the surface of carrier particles by an electric force caused by friction therebetween.
When a two component developer approaches an electrostatic latent image, toner particles in the developer are attracted to the electrostatic latent image by the electric field formed by the electrostatic latent image because the electric field overcomes the bonding force of the toner particles, thereby forming a visual image (toner image) on the electrostatic latent image. This developer is repeatedly used while the toner is added thereto to compensate for the toner used for development.
Therefore, carrier is required to have such a surface as not to be easily contaminated with toner, which has a charge with a polarity opposite to that of the carrier, so that the carrier can be used for a long period of time while maintaining the frictional charging ability thereof.
However, due to collision of carrier particles and mechanical agitation in a developing device, and generation of heat caused by such collision and agitation, a spent toner problem in that the toner is fixed to the surface of the carrier particles and thereby the charging ability of the carrier particles is deteriorated with time is caused.
In attempting to prolong the life of carrier by preventing occurrence of the spent toner problem, published unexamined Japanese patent applications Nos. (hereinafter referred to as JP-As) 55-127569, 55-157751, 56-140358, 57-96355, 57-96356, 58-207054, 61-110161 and 62-273576 have proposed techniques such that the surface of core particles of carrier is coated with a resin having a low surface energy such as fluorine-containing resins and silicone resins.
Specifically, JP-A 55-127569 discloses a carrier covered with a layer including a room temperature crosslinking silicone resin and a positively chargeable nitrogen-containing resin. JP-A 55-157751 discloses a carrier coated with a material including at least one kind of modified silicone resin. JP-A 56-140358 discloses a carrier covered with a resin layer including a room temperature crosslinking silicone resin and a styrene-acrylic resin. JP-A 57-96355 discloses a carrier prepared by forming two or more cover layers on the surface of core particles, wherein the cover layers have poor adhesiveness to each other at the interfaces therebetween. JP-A 57-96356 discloses a carrier covered with a layer including a polyvinyl acetal resin crosslinked by an isocyanate. JP-A 58-207054 discloses a carrier having a cover layer including a silicone resin including silicon carbide as a filler. JP-A 61-110161 discloses a positively chargeable carrier having a cover layer including a material having a critical surface energy of not higher than 20 dyne/cm. JP-A 62-273576 discloses a developer including a carrier coated with a material including a fluorinated alkyl(meth)acrylate copolymer and a toner including a chromium-containing azo dye as a charge controlling agent.
Recently, there is an increasing need for high quality images in electrophotography, and therefore the particle diameter of toner used for the electrophotographic developer becomes smaller and smaller. In addition, the image recording speed becomes faster and faster. Therefore, the above-mentioned spent toner problem tends to be easily caused. Further, when toner including a wax is used for developer to enhance the maintenance property thereof, a large amount of spent toner is produced, thereby easily causing a charging problem in that the charge quantity of the toner deteriorates; a toner scattering problem in that toner scatters around a developing device; and a background development problem in that background area of an image is soiled with toner particles.
In order to form a cover layer having good durability on a carrier, it is effective to fixedly adhere a resin having a relatively low surface energy to the surface of a core material. However, resins having a low surface energy generally have poor adhesiveness to core materials. In order to fixedly adhere a resin to core particles, a technique, which is used for TEFLON coating and in which the coated resin is heated at a high temperature or crosslinked, can be preferably used. However, when core materials used for carrier are heated to a temperature higher than the Curie points thereof, the core materials lose their magnetism. Therefore, it is hard to use such a technique for carrier.
When a developer used for electrophotographic full color image forming apparatus causes the spent toner problem and/or a problem in that the cover layer of the coated carrier is abraded or peeled, the resistance of the carrier and the amount of carrier supplied to the development region change, thereby changing the image qualities such as image density (particularly the density of highlighted portions). In addition, when the cover layer is ground and thereby the filler included in the cover layer is mixed with a color toner (particularly yellow toner), the color tone of the color toner is seriously changed, resulting in deterioration of image qualities.
Further, when a carrier is used while the environmental conditions are changed, a problem in that the charge quantity of the carrier changes, resulting in change of image qualities such as image density is often caused.
Because of these reasons, a need exists for a carrier, which can maintain a good chargeability for a long period of time and which can stably produce high quality images with hardly causing the above-mentioned problems such as the spent toner problem, background development problem and toner scattering problem even when the environmental conditions change.